1. Field of the Invention
The invention generally relates to surgical implements, and more particularly, to medical tensioning and retaining systems.
2. Description of the Related Art
Medical fixation of skeletal structures is an important procedure for orthopedic surgeons. Surgeons use fixation systems in a variety of applications, including spine fusions, hip arthroplasty, fracture fixation, sternum closures, and the like. Certain fixation systems, such as cerclage systems, include a wire, cable, or suture wrapped tightly around the relevant bone or other structure and fixed in place. The cable usually comprises a wire or cable of biocompatible material, which is fixed in place by either twisting the free ends together or applying a retainer after an appropriate amount of tension has been placed on the wire or cable. The most common retainer is a sleeve which is crimped onto the wire or cable.
Achieving the appropriate tension is vital to the proper functioning of the fixation system. For example, excessive tension in a cerclage cable can cause cable failure, bone fracture, or avascular necrosis of the bone around which the cable is wound. On the other hand, insufficient tensioning prevents the system from performing properly; sufficient tension must be maintained for proper fixation. Furthermore, the tensioning process is often difficult due to the small components and the high tensions required.
Various systems are available for applying and measuring cable tension. For example, both the Howmedica Dall Miles System and DePuy Control Cable System offer force-multiplying pliers to apply and measure cable tension. To facilitate cable tension measurement, the Howmedica system uses a beam deflection torque wrench connected to the drive mechanism of the pliers, and the DePuy system uses a tension gauge built into the tensioning pliers. Each of these systems, however, only measures tension before the cable is secured. After securing the cable, for example by crimping a cable retainer, cable tension frequently decreases dramatically due to inadvertent oblique loading during the crimping process. In addition, after the implantation of the tensioning system, post-operative shifting of bone fragments and slippage of the cable in the securing mechanism may also contribute to loss of cable tension.
A tensioning system according to various aspects of the present invention provides dynamic tensioning to maintain high tension in a fixation system despite tissue shifting, cable slippage, or other inadvertent loss of tension. In particular, an exemplary embodiment of an in-line dynamic tensioning system includes a dynamically tensioning cable retainer, suitably comprising a suture anchor or bone screw, which provides a bias to maintain tension in the cable after installation. The retainer includes a retaining mechanism for retaining the cable or suture and a biasing mechanism connected to the retaining mechanism. In the event of cable slippage or slackening, the biasing mechanism tends to take up the slack and maintain tension. As a result, cable tension is maintained regardless of inadvertent cable slackening after installation.